Is a ‘shockingly strange’ reason why mosquitoes always find us

(ORDO NEWS) — The relentless precision with which some species of mosquitoes track down humans may be due to their bizarre olfactory systems, which have a built-in reserve for detecting human scents.

Mosquitoes smell CO2 or human sweat using unique chemoreceptors in their antennae and maxillary palp, a jointed sensory appendage of insects.

A new study led by scientists from Boston University and Rockefeller University explains why mosquitoes sense us so well, even when researchers genetically disable human-specific chemoreceptors.

According to the study, at least one species of mosquito, Aedes aegypti, has a very different way of organizing its olfactory system compared to most animals.

Using CRISPR as a gene-editing tool, the researchers created mosquitoes whose olfactory neurons expressed fluorescent proteins and glowed under a microscope when certain odors were nearby. This allowed the researchers to see how different scents stimulate the olfactory system.

It turned out that A. aegypti connects several olfactory sensory receptors to one neuron – this process is called co-expression.

According to this team, this reverses the basic principle of the science of smell, which states that each neuron has only one chemoreceptor associated with it.

“It’s shockingly strange,” says Boston University neuroscientist and senior study author Meg Younger. “It’s not what we expected.”

“The main dogma in smell is that sensory neurons, for example in our nose, express one type of olfactory receptor,” says Younger.

This axiom holds true for the honey bee (Apis mellifera), tobacco worm (Manduca sexta), and fruit flies (Drosophila melanogaster), which have roughly the same number of chemosensory receptors in their olfactory glomeruli (glomeruli are spherical structures in the brain that receive olfactory signals).

However, A. aegypti has twice as many receptors as the glomerulus, a “striking disparity,” the researchers write.

These results suggest an unconventional olfactory system that expresses multiple sensory receptors in individual neurons.

“The redundancy provided by the olfactory system… may increase the strength of the mosquito olfactory system and explain our long-standing failure to disrupt mosquito human detection,” the researchers conclude.

The lure of blood food is strong, as female mosquitoes must feed on human or animal blood in order to reproduce.

The long-term goal of the research is to create improved mosquito repellents that effectively hide human odor, or to develop attractants that distract mosquitoes from food.

The ability of mosquitoes to find a person makes them active carriers of viral diseases such as dengue, zika, yellow fever and chikungunya. Together, these viruses kill about 700,000 people each year.

“By learning about how smell is encoded in their olfactory system, we can create compounds that are more effective based on their biology,” says Younger.

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